border-1px($color)
  position: relative
  &:after
    position: absolute
    left: 0
    bottom: 0
    border-bottom: 1px solid $color
    width: 100%
    content: ""

border-none()
  &:after
    display none

bg-image($url)
  background-image: url($url + '@2x.png')

@media (-webkit-min-device-pixel-ratio: 3),(min-device-pixel-ratio: 3)
  background-image: url($url + '@3x.png')
